As an architectural feature, a window provides daylight to an interior space and allows the building occupants a view to the outside. When direct beam solar radiation falls directly on a window, the light that enters has an intensity of several hundred watts per square meter and is generally too bright to be used directly as illumination. The light must be attenuated, diffused, or reflected onto the ceiling and walls of the room by a window treatment to provide comfortable illumination. Daylight harvesting systems are now commonly employed which automatically dim or turn off lighting in the vicinity of windows when natural light is available to reduce energy consumption and building heat load.
Typical solutions for attenuation of incoming sunlight include opaque or translucent shades, blinds, and curtains. These can reflect a portion of the incoming solar radiation to reduce light levels and glare, but have the disadvantage of having only coarse controllability and do not provide illumination to the area away from the window deeper into the room. More sophisticated blinds and fixed reflecting louvers are available that can reflect light up towards the ceiling to bring light further into the room, but the degree of illumination is not directly controllable.
Another common solution to handling the solar energy on a vertical window is coatings and films that change the optical properties of the window glazing to either reflect or absorb selective bands of the visible and infrared spectrum. These have the advantage of reducing the need for internal window treatments, but these are typically permanent changes to the window characteristics and so they permanently reduce the amount of solar energy available for useful illumination and heating. There is considerable research and development in windows with electrochromic coatings that allow direct control of the transmissivity of the glazing. These currently suffer from high cost and slow reaction time.
There is believed to be a window treatment commercially available in Europe that allows the user to selectively prefer heating or lighting, but the product does not provide for complete reflection of a portion of unwanted solar energy.
Moreover, the solar radiation into the side windows of a building is present for only a few hours of the day—either morning, noon, or afternoon. To take best advantage of this intermittent heat source, it is common practice in passive solar heating design to include some type of thermal storage so that the heat gathered over, for example, three or four hours can be spread over a longer period to avoid overheating during the sun periods and to provide comfort for hours afterwards.
Typical window shades block or absorb sunlight and convert the sunlight into heat on the shades which is brought into the room by thermal convection. The temperature of the air that rises from the back of the window shade is typically only 10° or 15° warmer than the room air. This provides little temperature differential to drive thermal storage. A very large mass is required to store a significant amount of heat with such a small temperature difference. Therefore, typical window shades and blinds have very little ability to store any of the heat and therefore the heat that they do provide to the room is highly variable in a function solely of the heat input through the window.
Therefore, there remains a need in the art of solar energy management systems to simultaneously provide for the control of lighting and temperature in a room that is easy to manufacture and deploy and that reliably manages both lighting and temperature conditions over an extended period and in varied conditions (such as varying sunlight conditions).